a) Field of the Invention
This invention relates to a spring damper device for use in a lockup clutch of a torque converter.
b) Description of the Related Art
FIG. 1 is a cross-sectional view showing a basic construction of a torque converter equipped with a lockup clutch. FIG. 2 is an illustration as viewed in the direction of arrows II--II of FIG. 1. In these drawings, there are shown a torque converter main body 10, a centering portion 1 of an engine power output shaft, a pump 2, a turbine 3, a stator 4, a turbine hub 5 of the torque converter, an axis 6, a power input casing 21 connecting the engine power output shaft and the pump 2 with each other, a piston 22 axially movable on the hub 5 and acting as a clutch plate, a retainer plate 23 fixed on the piston 22 by rivets 26, and tabs 24 located inside a spring 32 which is arranged on an inner peripheral side of an outer peripheral flange 28 of the piston 22. These tabs 24 have been formed by radially slitting a peripheral portion of the retainer plate 23 at predetermined angular intervals and bending out every second slitted peripheral portions. Designated at numeral 25 are spring end face support members which are located radially outside the retainer plate 23 and are bent into turned square U-shapes with the open faces of the Us rendered broader. Numeral 33 indicates caps in which end portions of the corresponding springs 32 are received, respectively. Numeral 27 indicates a friction member of the clutch.
A driven plate 31 is arranged opposite the end face supporting members 25. Incidentally, a portion indicated by sign 32A in an upper part of FIG. 2 indicates that the spring 32 has been compressed, its caps 33 have already been brought into contact with each other and the spring 32 cannot be compressed any further. On the other hand, at another portion indicated by sign 32B in a lower part of FIG. 2, the tab 24 is omitted, and the spring and its associated caps are shown in cross-section.
A description will next be made about the operation of the torque converter equipped with the lockup clutch. Now assume that a hydraulic pressure is applied on a right side of the piston 22. Then the piston 22 is moved leftwards and is brought into contact under pressure with the power input casing 21. Rotation of the piston 22 is hence transmitted to the turbine 3 via the end face supporting members 25, the caps 33, the springs 32 and the driven plate 31. As the turbine 3 is fixedly mounted on the turbine hub 5, a power output shaft (not shown) is rotated by way of an unillustrated power output hub.
Variations in torque are absorbed at the springs 32. Each spring 32 is positioned between its associated end face supporting members 25 and is also prevented from axial dislocation by the associated tab 24.
Further, as is illustrated in FIG. 3, it has also been contemplated to form each spring 32, which is arranged for the absorption of torque variations, as a double coil spring formed of an inner spring 32X and an outer spring 32Y and further to make the inner spring 32X shorter to delay its actuation, thereby providing the double coil spring with two-step damping characteristics. There is also a modification that linear cylindrical springs are set in the form of arcs of a circle to save a space for their arrangement.
In the case of double coil springs such that, as is shown in FIG. 3, each outer spring 32Y is inserted between spacers 34, which are in turn positioned between the associated two end face supporting members 25, and the corresponding shorter inner spring 32X is inserted in the outer spring 32Y, it is designed to hold each outer spring 32Y in a curved form or to allow each outer spring 32Y to flex into a curved form under a centrifugal force during rotation. In such a conventional device, however, an inconvenience arises as shown in FIG. 4 and FIG. 5 which is an enlarged view of a part A of FIG. 4. Namely, opposite end portions of each linear inner spring 32X may enter spaces between pitches of the associated outer spring 32Y during an operation so that the opposite end portions of the linear inner spring 32X may ride on the associated outer spring 32Y. This leads to unstable damping characteristics and hence to a shorter service life.